1. Field of the Invention
The present invention relates to a data decoding apparatus and method applied to a digital recording medium playback apparatus such as an optical disk (disc) or magnetic disk, and more particularly to a data decoding apparatus and method that uses Viterbi decoding in according with a decoding trellis based on the characteristics of the recorded signal to improve the playback performance with playing back the recorded data from the recording medium.
2. Description of the Related Art
In recent years, in recording and playback apparatuses for digital data such as dealt with on digital recording media such as optical and magnetic disks, with the advent of high-density recording of information on a recording medium, playback by the partial response (PR) method has come into use, and Viterbi decoders are commonly used to decode a signal.
When employing the PR method, there are such advantages as being able to maximum the density over a limited signal transmission bandwidth, and when a Viterbi decoder is used it is possible to achieve decoding with superior S/N ratio and low code error rate while making the maximum utilization of the information contained in the playback signal.
In a playback apparatus for a digital recording medium such an optical or magnetic disk such as noted above, asymmetry is know to occur in the playback signal as a result of the recorded pit length and depth. For this reason, in a decoding apparatus in such a playback apparatus, a means for controlling the slice position of the playback signal detection circuit (for example, an autoslicer) is used, this control being based on the asymmetry of the playback signal.
In the Japanese laid-open patent application publication H6-150549 there is disclosure of an optical information playback apparatus which determines compensation information for a Viterbi decoder from an area into which the bit information has been recorded, so as to accommodate asymmetry such as noted above in the playback signal.
With the increase in recording density in recent years, however, the non-linear influence of a prior pit or a land on a following bit has increased to a degree that cannot be neglected, so that the simple asymmetry (linear component) compensation of the past is not sufficient.
Additionally, the compensation of such asymmetrical components is expected to exceed the capabilities of either a waveform equalizing circuit used to perform playback signal compensation or Viterbi decoding for the purpose of accommodating a deterioration in the signal-to-noise ratio of a playback signal attributed to an increase in recording density.
It is an object of the present invention to provide a data decoding apparatus and data decoding method capable of reducing the non-linear influence of a prior pit or a land on a following bit attributed to increased recording density, and of perform high-quality decoding of even bits that are affected by fluctuations in the playback signal amplitude.
To achieve the above-noted object of solving the problems inherent in the related art, a data decoding apparatus according to the present invention uses Viterbi decoding in accordance with a code trellis based on the characteristics of the recorded code to perform decoding of data that has been recorded onto a recording medium and played back therefrom, and has a run length detection means that detects the run length of the recorded data.
In order to achieve the above-noted object, another aspect of a data decoding apparatus according to the present invention has a target value varying means for varying the target value for performing metric calculation of the Viterbi decoding, based on the run length information detected by the run length detection means.
In yet another aspect of a data decoding apparatus for the purpose of achieving the above-noted object, the run length detection means is formed by a counter to count the run length, and the target value varying means is a monitoring means for protecting against faulty operation caused by using a count value that should not be used as an information source.
More specifically, according to the present invention, in a pathmetric calculation unit when performing Viterbi decoding according to a decoding trellis, by providing a means for detecting a prior pit or land length and a means for varying the target value of the metric calculation based on this detection, even in a case in which, because of an improvement in the recording density, a prior pit or land length has a large effect on a subsequent bit (that is, a bit that is affected by fluctuation in the playback signal amplitude), it is possible to perform signal playback (decoding) under good conditions at all times.
From yet another aspect of the present invention, there is provided a data decoding method of performing decoding of recorded data played back from a recording medium, using Viterbi decoding in accordance with a decoding trellis based on a recorded signal characteristic, comprising the step of:
detecting a run length of recorded data at pathmetric calculating.